The major objective of this proposal is to test the hypothesis that thyroid hormones influence fetal lung growth and maturation, and pulmonary function in the normal and distressed neonate. The investigations are designed to test the following hypotheses: 1) That iodothyronines influence fetal lung growth, 2) that iodothyronines influence pulmonary surfactant maturation synthesis, 3) that iodothyronines influence pulmonary function in distressed and normal preterm and full-term neonates, and 4) that the methods used to determine metabolic clearance and secretion rates for thyroid hormones are valid. Initial studies conducted in the surgically thyroidectomized ovine fetus have confirmed that lack of iodothyronines during the last trimester significantly delayed lung maturation. The next series of studies will investigate the influence of thyroxine infusion on ovine lung maturation and surfactant synthesis. BIBLIOGRAPHIC REFERENCE: Erenberg, A., Rhodes, M.L., Weinstein, M.M. and Kennedy, R.L.: Thyroid hormones and ovine lung maturation. Clin. Res. 25:193A, 1977.